When it comes to crystal filters, it appears
quickly that you have to pay a high price if you want good performance
devices : so I designed them by myself using methods proposed by
W. Hayward W7ZOI, J. Makhinson N6NWP and others.
I
built 3 types of ladder crystal filters : one for SSB (2300 Hz), the
other for CW (400 Hz) and the third as a noise filter. As all the
modules of my home-made transceiver have 50 ohms input and output
impedances, the filters include matching transformers.
First I bought a batch of 30 x 8 MHz crystals ( # $1 each ) and sorted them out by frequencies and Q using a simple test fixture. I selected 12 crystals to design an SSB filter : their main characteristics are summarized below :
Fm = 7996.582 kHz Qm = 102000 Lm = 19.5 mH
I then selected a 2.3 kHz Chebycheff 0.1 dB design and, after many simulations with the W7ZOI programs, was ready to build the prototype : its schematic is presented below.
SSB Filter Frequency Response.
Some
measured characteristics : BW(-3dB) = 2300 Hz, BandPass Ripple< 1
dB, Insertion Loss 6 dB, Ultimate Rejection # 100dB, 6/80dB Shape factor # 1.6
This
picture shows the inside of the SSB ladder filter : its box is fairly
large because first tests show that the same circuit, housed in a
smaller box, has a much lower ultimate rejection.
Using the same method, I then designed and build a 400 Hz Butterworth Filter and a 2700 Hz noise filter ( schematics below) with some of the remaining crystals :
Measured characteristics : BW(-3dB) = 360 Hz, Ultimate Rejection < 90dB, 6/60 dB Shape factor # 3.2
6 Poles CW Filter.
Measured characteristics : BW(-3dB) = 2700 Hz, Ultimate Rejection < 60dB, 6/40 dB Shape factor # 2.6